Thermoplastic films based on blends of polypropylene and polybutene

ABSTRACT

A biaxially oriented thermoplastic film structure formed from a resinous blend comprising polypropylene and polybutene, polybutene being present in an amount of more than 10% but less than 20% by weight, based upon the total weight of the blend.

United States Patent n 1 Schard [451 Aug. 19, 1975 THERMOPLASTIC FILMS BASED ON BLENDS OF POLYPROPYLENE AND POLYBUTENE Malcolm P. Schard, Califon, NJ.

Mobil Oil Corporation, New York, NY.

Filed: May 7, 1973 Appl. No.: 357,754

Inventor:

Assignee:

US. Cl 260/897 A; 264/289; 264/290 Int. Cl. C08f 29/12 Field of Search 260/897 A References Cited UNITED STATES PATENTS 12/l967 Musso et al 260/897 FOREIGN PATENTS OR APPLICATIONS 835,330 5/1960 United Kingdom 260/897 775,184 2/1968 Canada 260/897 Primary Examiner-John C. Bleutge Assistant ExaminerC. J. Seccuro Attorney, Agent, or FirmCharles A. Huggett; James D. Tierney 5 7 ABSTRACT 3 Claims, No Drawings THERMOPLASTIC FILMS BASED ON BLENDS OF POLYPROPYLENE AND POLYBUTENE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to blends of thermoplastic materials and oriented shrink films produced therefrom having improved properties. More particularly, the present invention relates to blends of polypropylene resin and polybutene-1 resin wherein said polybutene is present in an amount of more than but less than based upon the total weight of the resinous blend mixture. Such resinous blends have been found especially useful in the fabrication of shrink films having unexpectedly excellent heat seal characteristics as well as exhibiting good optical properties.

2. Description of the Prior Art In the past a wide variety of resinous thermoplastic materials have been employed in the manufacture of heat shrinkable films including oriented polyethylene which has been irradiated and oriented polypropylene for example. Additionally, U.S. Pat. Nos. 3,634,552 and 3,634,553 disclose the employment of resinous blend mixtures suitable for the production of heat shrink films. These patents disclose mixtures such as polypropylene blended with varying amounts of an ethylene-butene-l copolymer resin containing minor amounts of ethylene. Canadian Pat. No. 775,184 discloses the employment of blends of polypropylene and polybutene-1, such blends, according to this patent, being suitable for the production of biaxially oriented films. U.S. Pat. No. 3,356,765 discloses the employment of blends of polypropylene and polybutene-1 as suitable for producing relatively thick sheets, i.e., thicker than a packaging film material which is usually on the order of about one to two mils or less.

SUMMARY OF THE INVENTION It has now been found that certain properties including heat seal strength and optical clarity of shrink films may be improved by fabricating such films from resinous blends of polypropylene and polybutene, polybutene being present in a minor amount of more than 10% but less than 20% by weight based upon the total weight of the blend. In a more specific aspect, the polybutene content of the blend composition comprises an amount, in the range of about 12 to about 18% by weight, sufficient to impart to film prepared from said blend higher heat seal strength than a comparable blend but containing 10 or 20% of polybutene while retaining good optical properties. It has been found that by controlling the amount of polybutene present in such blends, within a relatively narrow range, that shrink films may be produced exhibiting improved properties.

DESCRIPTION OF SPECIFIC EMBODIMENTS The polypropylene resin component of the shrink film compositions in accord with the present invention is a highly isotactic polypropylene, i.e, for example, a polypropylene having a solubility isotactic index of at least about 90 as measured in heptane. Such polypropylene is produced, as is well-known in the art, by polymerizing propylene in the presence of stereospecific catalyst systems. Such polypropylene homopolymers include those which are formed in the presence of minor amounts of ethylene monomers, i.e., on the order of less than 5% by weight added to facilitate filmforming techniques and film properties formed from such resins.

The polybutene-l resins which may be employed in the resinous blend compositions, in general, have an isotactic index (weight percent insoluble in diethyl ether) of about or higher and usually about or more. A procedure which may be employed to produce isotactic polybutene-1 suitable for employment in the present invention is fully described in U.S. Pat. No. 3,362,940. Examples of polybutene resins which are particularly suitable for employment in the resinous blend compositions to produce films in accord with the present invention are those exhibiting the following ranges of physical properties.

Elongation (7:) 300 (minimum) A number of prior art orientation techniques may be employed to stretch and form the shrink film structures of the present invention. In general, oriented films may be obtained by stretching processes in which tensions capable of stretching the film are applied to the film, the directions of which form an angle of about 90. These film stretching tensions to orient the film may be applied sequentially to produce a biaxially oriented film as when a film is stretched initially in one direction and thereafter stretched in a direction 90 divergent from the first stretching direction. Also, the film may be biaxially stretched and oriented simultaneously whereby longitudinal and transverse tensions are applied to the film at the same time resulting in a simultaneous longitudinal and transverse stretching of the film. Such latter processes are well known in the art and include, for example, the double-bubble" method which comprises extrusion of a tubular base film, cooling of the tubular film, reheating and inflation of the tube and simultaneously drawing the inflated tube in a longitudinal direction thereby imparting biaxial orientation to the film. A common method and apparatus for the sequential biaxial orientation of film comprises passing the film sheet through a series of rotating draw rollers which impart longitudinal direction stretch and subsequently transversely drawing the longitudinally stretched film, for example, by passing it through a heated tenter frame wherein the film is stretched in a transverse direction.

Prior to extrusion into a film structure, the polypropylene and polybutene-1 components comprising the blend composition of the present invention are blended together to form a substantially homogenous resin mixture. This may be accomplished, for example, by mastieating the components on a slightly warm, differential speed, two-roll mill or in similar polymer blending machinery, such as a Banbury mill. The blend is then extruded into a film utilizing a standard extruder and tubular or flat film die and is subsequently oriented utilizing any one of a number of prior art film orientation vention and the gauge may generally vary from about 0.10 mil up to about 10 mils and preferably from about 0.5 mil to about 2.0 mils depending to a great extent on the type of shrink film packaging applications for which the film is manufactured.

EXAMPLE I A mixture of (a) 85% by weight of polypropylene a high isotaetic content resin having a melt index at lers. The biaxially oriented tube was then passed to a set of wind-up rollers.

in the following Table 1, physical properties are given for the shrink film, i.e., Blend B, produced in accordance with Example 1. For comparative purposes, also reported in Table 1 are the physical properties of two other shrink films (i.e., Blend A and Blend C), formed in accordance with the procedure of Example 1, from resinous blend compositions outside the scope of the v 53 3 i (,7) 15 welght of polybutene'l present invention. All films were 0.75 mil 1n thlckness. t e O Owmg propemes' Heat sealing may be effected by any of the conventional means known in the art, including hot wire, bar

Melt Index 1.8 Melting Poi, 121C and d electric. Depending upon various factors mclud (differential thermal mg thickness and type of sealer, dwell time is generally li M St h I587 about 0.52 seconds and sealing temperature is 300 to '8 [Eng 400F. 1n the following Table 1 heat seals were formed Ten ile Break Strength 391 utilizing a standard hot wire impulse sealer. An impulse .s.1.) D cnsity (g/cc) 0906 of 125 volts was applied across a 0.0036 inch diameter Elongation (percent) 411 nichrome wire for an lmpulse dwell time of 0.2 second and a sealing jaw dwell time of 0.9 second. The applied It was blended utilizing a Banbury mill and subsepressure during sealing was about 20 p.s.i. Heat seal q y fed into the pp of a Standard. rotating strength data presented in the following Table was descrew, extrusion apparatus h h erv o f r h r mix termined utilizing a standard tensile testing machine, in and melt the blend. The temperature of the melt within accordance with A.S.T.M. Designation D 882- TABLE 1 Blend A Blend B Blend C "/1 Polybutenc-l 10 15 20 /r Polypropylene 90 85 X0 'lcnsile Modulus (psi) *MD 241,500 203,000 199,000 TD 237,800 187,000 184000 Tensile Strength at Break MD 21.010 23.100 21800 (psi) TD 24,000 19,900 20,100 71E1ongation at Break MD 49 53 54 TD 8 5s 84 Haze. /l (A.S.T.M. D-1003) 1.5 1.6 2.0 Gloss, /r at (A.S.T.M. MD 874 85.9 83.7 D-2457-65T) TD 87.0 84.8 82.6 Coefficient of Friction 0.42 0.39 0.39 (A.S.T.M. D 1894-63) "/1 Shrink at 130C. 408 40.9 39.7 Shrink Force at 95C. (psi) MD 345 326 286 TD 400 338 359 Seal Strength (psi) MD 6.770 13,500 10.600 lD 5,640 1 1.500 10.800

MD Machine Direction TI) r: Transverse Direction the extruder was maintained at about 490F. The blend It an be seen from the data presented in foregoing was subsequently extruded in the shape of a tube from Table 1 that the resinous blend compositions of the a tubular die affixed to the outlet end of the extruder, present i tio e Blend B, containing more than the die being maintained at a temperature of about 10% polybutene and less than 20% thereof (eg, 5%) 450F. The tube was quenched to a temperature of exhibits acombination of good haze properties and exabout 140F., which was substantially below the crysceptionally high seal strengths as contrasted to films talline melting points of the resinous components of the fabricated from blends of polypropylene containing film, immediately upon emergence from the die. The 10% polybutene-l or 20% polybutene-l by weight. As extruded tube had an external diameter of about 4 /8 shown by the data contained in Table 1 when films are inch and a wall thickness of about 25 mils. Upon coolis fabricated from blends containing 10% by weight of ing the tube was collapsed by a set of nip rollers and polybutene-l (i.e., Blend A) such films, although they passed into a preheat oven wherein it was reheated. possess acceptable haze characteristics, exhibit unde- The temperature of the preheat oven was maintained sirably low heat-seal strengths. Increasing the amount at about 405F. in the entry Zone thereof and at approxof polybutene-l to the 20% by weight level (i.e., Blend imately 1,050F. at the exit zone of the oven with the C) results in a film having somewhat improved heat exiting tube having a surface temperatur f b t seal properties, however, the haze level is increased to 275F. The collapsed, heated tube was immediately rethe poin to m ke ch films undesirable for certain inflated with air under pressure which expanded the p gi g pp ications her the See-through charhcated tube by a ratio of about 5.9:1 in a transverse diilCtBriStiCS 0f the film are most i porta t. A Shown in rection and a substantially similar ratio in the longitudi- Table films Produced from blends comammg more nal direction. The expanded tube was subsequently collapsed by a pair of draw rollers operating at a rotational speed higher than the rotational speed of said nip rolthan 10% polybutene and less than 20% thereof by weight, such as Blend B which contains 15% by weight of polybutene-l, exhibit unexpectedly high heat seal strengths. Such seal strengths are unexpectedly greater than those obtained from Blends A and C, and by weight of polybutene-1 respectively and, additionally, the haze characteristics of the films of Blend B make such films ideally suited for packaging applications requiring high transparency.

Additives to impart desired film surface properties may be incorporated into the polymer blend compositions of the present invention, for example, additives which reduce or eliminate the films tendency to fog with condensed moisture from emitting commodities such as fresh meats and produce. Also, antistatic additives which reduce the tendency of the film to build up static electricity charges and many others such as anticling additives; additives which promote film slip; and additives which reduce the tendency of multiple layers of film to block together may be added to the blend compositions.

Although the present invention has been described with preferred embodiments, it is to be understood that modificatons and variations may be resorted to Without departing from the spirit and scope of this invention as those skilled in the art will readily understand.

What is claimed is:

l. A heat shrinkable biaxially oriented thermoplastic film comprising a resinous mixture containing:

a. from about 82% to about 88% by weight of a high isotactic content polypropylene resin; and b. from about 12% to about 18% by weight of a high isotactic content polybutene resin;

said film having a thickness of from about 0.10 mil up to about 10 mils, said thermoplastic film being further characterized in that it has improved optical properties in comparison to film containing 20% by weight of said polybutene resin in said mixture and improved heat seal strength characteristics in comparison to films containing 10% by weight of said polybutene in said mixture.

2. A film as defined in claim 1 which comprises a biaxially oriented film tube.

3. A film as defined in claim 1 which comprises about 85% by weight of said polypropylene and about 15% by weight of said polybutene. 

1. A HEAT SHRINKABLE BIXIALLY ORINTED THEROMPLASTIC FILM COMPRISING A RESINOUS MIXTURE CONTAINING: A. FROM ABOUT 82% TO ABOUT 88% BY WEIGHT OF A HIGH ISOTACTIC CONTENT POLYPROPYLENE RESIN, AND B. FROM ABOUT 12% TO ABOUT 18% BY WEIGHT OF A HIGH ISOTACTIC CONTENT POLYBUTENE RESIN, SAID FILM HAVING A THICKNESS OF FROM ABOUT 0.10 MIL UP TO ABOUT 10 MILS, SAID THEROMPLASTIC FILM BEING FURTHER CHARACTERIZED IN THAT IT HAS IMPROVED OPTICAL PROPERTIES IN COMPARISON TO FILM CONTAINING 20% BY WEIGHT OF SAID POLYBUTENE RESIN IN SAID MIXTURE AND IMPROVED HEAT SEAL STRENGTH CHARACTERISTICS IN COMPARSION TO FILMS CONTAINING 10% BY WEIGHT OF SAID POLYBUTENE IN SAID MIXTURE.
 2. A film as defined in claim 1 which comprises a biaxially oriented film tube.
 3. A film as defined in claim 1 which comprises about 85% by weight of said polypropylene and about 15% by weight of said polybutene. 